The present invention relates to a semiconductor device and a method for fabricating the semiconductor device.
InGaP is able to epitaxially grow on GaAs and has a wider energy band gap than GaAs. These characteristics are utilized for InGaP to be applied to various devices.
For example, in a HEMT (High Electron Mobility Transistor), InGaP is used as an electron donating layer for donating electrons to a channel layer, and in an HBT (Hetero-junction Bipolar Transistor), InGaP is used as an emitter layer, which requires a wide band gap.
InGaP has good etching selectivity with respect to compound semiconductor layers containing As element, such as GaAs, AlGaAs, InGaAs, etc. and is widely used in semiconductor devices having structures requiring selective processing.
For example, the specification of Japanese Patent No. 2,581,452 discloses a semiconductor device including an electron donating layer of an InGaP layer formed on a channel layer of a GaAs layer or an InGaAs layer. The semiconductor device described in the specification of Japanese Patent No. 2,581,452 will be explained with reference to FIG. 10.
A buffer layer 102 of GaAs or AlGaAs is formed on a semi-insulating GaAs substrate 10. An electron donating layer 104 of AlGaAs doped with an n-type impurity is formed on the buffer layer 102. A channel layer 106 of GaAs or InGaAs is formed on the electron donating layer 104. An electron donating layer 108 of InGaP is formed on the channel layer 106. On the electron donating layer 108, a lower contact layer 110 of AlGaAs, an intermediate contact layer 112 of InGaP and an upper contact layer 114 of GaAs are formed. A recess region 116 which reaches the electron donating layer 108 is formed in the contact layers 110, 112, 114, and a gate electrode 118 is formed on the exposed electron donating layer 108. A source electrode 120 and a drain electrode 122 are formed on the upper contact layer 114.
As described above, in the semiconductor device described in the specification of Japanese Patent No. 2,581,452, the electron donating layer 108 is formed of InGaP, which has a wider band gap than the channel layer 106 and has good etching selectivity with respect to AlGaAs forming the lower contact layer 110, whereby the semiconductor device can be a HEMT having high performance, homogeneity and reproducibility.
However, in the conventional structure of the semiconductor device, which includes the AlGaAs layer or the GaAs layer on the InGaP layer, resistance in the interface between the InGaP layer and the AlGaAs layer or the GaAs layer is high, which is a cause for high source resistance. This is a new finding by the inventor of the p resent application. This affection is conspicuous especially in semiconductor devices of the structure formed by planar doping, which includes, for higher gate voltage resistance, a doped layer provided at an intermediate part of an electron donating layer 108. These affections are also observed in semiconductor devices including an AlGaAs layer or an GaAs layer on an AlGaInP layer.
Causes for such increase of resistance in the interface between the InGaP layer or the AlGaInP layer and the AlGaAs layer or the GaAs layer have not yet been clarified, and it has been required to make clear these causes.